TRANSPORTED ROCK LEDGES. 83 



seems necessary to call in the action of the ice sheet to account for these 

 transportations, for they can not, as a rule, be the result of landslides or of 

 a slow creeping down the slope. Hershey refers them to the closing stage 

 of the ice movement, for the reason that if transportation had occurred in 

 the midst of the invasion the ledges would, in all probability, have been 

 more thoroughly intermingled with the drift. Except in a very few 

 instances no drift pebbles have been found in the body of these deposits. 

 There are often bowlders and smaller drift pebbles scattered over their 

 surface. 



Hershey has also discovered places where the ledges have been pressed 

 into slight folds without suffering transportation. In these folds the strata 

 show dips of 10 to 30 degrees, while the greatest dips of the strata due to 

 orographic bending, so far as he has discovered, do not exceed 2 degrees 

 The reference of this disturbance to ice action appears well sustained. If 

 the folding of the strata is not due to this agency it seems necessary to 

 refer it to a remarkable local disturbance, since the strata in the surround- 

 ing districts are practically horizontal. 



The transported and disturbed rock masses are especially numerous in 

 the township of Dakota, in Stephenson County. Within 4 miles west and 

 southwest of the village of Dakota, Hershey has found at least thirty 

 distinct deposits of this class. They are generally conical or dome-shaped 

 masses a few rods in diameter, which appear as though embossed on the 

 top and slope of high rock ridges. When the internal structure is revealed 

 by excavations, the incoherent portions are found to alternate, both horizon- 

 tally and vertically, with other portions in which the original bedding- planes 

 have been but little disturbed. Hershey cites two instances of the occur- 

 rence of transported ledges in the midst of valleys 2 or 3 miles west of 

 Dakota, in which burrowing animals have broug'ht out water worn gravel 

 and sand from under the limestone. The largest ridge is about 75 feet high 

 and nearly obstructs the valley in which it stands. The smaller one is 

 about 30 feet high and is composed of Galena limestone, not much broken, 

 but with the strata dipping steeply in every direction from the center and 

 top of the mound. These masses are scattered widely over Stephenson 

 County east of the meridian of Freeport. 



Hershey has noted a tendency to greater development of this peculiar 

 class of rock transportation along' lines leading north and south from the 



